A colloquial, casual form of わからない meaning 'I don't know' or 'I don't understand.' Common in spoken Japanese.
そんなことは俺にはわからん。
I don't know about that.
先生の説明を聞いたが、まだよくわからん。
I listened to the teacher's explanation, but I still don't understand it well.
彼が何を考えているのか全くわからん。
I have no idea what he's thinking.